Passive voice is when the subject of the sentence is not the thing performing the action/verb. For instance, "a goal was scored" by itself doesn't tell you WHO the performer of the verb ("scored") is.
For example:
"The ball was thrown by Sarah." becomes
"Sarah threw the ball."
likewise:
"During the game, 3 goals were scored by Sarah" becomes
"During the game, Sarah scored 3 goals"